Velocity modulation tubes



Dec. 20, 1955 P. T. SMITH 2,728,019-

VELOCITY MODULATION TUBES Filed June 27, 1951 3 Sheets-Sheet l k 1.9M H 5 i 1; J 10 g 11 F i 21-22 4 07 J 2 0 19 7 14 c y lifl L10/47.: F. 7 i E A 5/0 L INVENTOR bit 0 52 14 Pg? Z'Smiib ORNEY 3 Sheets-Sheet 2 Filed June 27, 1951 1 m R ,.Y mm w M 1 I 1 Dec. 20, 1955 I P. T. SMITH VELOCITY MODULATION TUBES 3 Sheets-Sheet 5 Filed June 27, 1951 INVENTOR PJJJ'IJ' 7181211117 United States Patent VELOCITY MODULATION TUBES Philip T. Smith, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 27, 1951, Serial No. 233,795

14 Claims. (Cl. 315-7) This invention relates to electronic tubes, and particularly to velocity-modulation tubes, sometimes referred to as Klystrons.

Prior conventional types of velocity-modulation amplifiers and oscillators are limited, even under optimum operating conditions, to a theoretical maximum efficiency of about 58 per cent. It is a primary object of the present invention to substantially increase the output efliciency of velocity-modulation tubes.

In accordance with the present invention, the electron beam, either before or after its passage through the velocity-modulation region of the tube, passes through a region in which it is subjected to a transverse deflection field having the same frequency as the axial velocitymodulating field, and substantially the only electrons of the periodically deflected beam permitted to enter the drift region are those which are properly phased to contribute to the radio-frequency output of the tube.

In one form of the invention, the beam is velocitymodulated before deflection, whereas, in another form, it is velocity-modulated after deflection. In both forms, there is a partial barrier electrode in advance of the drift space to preclude passage thereinto to out-of-phase electrons which would produce a loss in output efliciency.

In accordance with another form of the invention as applied to a high power tube, the total collector current is divided between several parallel beams in individual cells providing separate drift regions to increase the allowable current density without encountering space charge limitations, the cell walls fixing the space potential along the beam and preventing the electrons from unduly lowering the space potential. More specifically, in a preferred high power arrangement, the beam cells are symmetrically disposed in an annular array about the longitudinal axis of the tube, with a common input cavity at one end of the tube and a common output cavity at the opposite end of the tube.

The invention further resides in methods and arrangements having the featuers of novelty and utility .hereinafter described and claimed.

For a more detail understanding of the invention, reference is made to the accompanying drawings, in which:

Fig. l is a schematic diagram of a conventional type of velocity-modulation tube;

Figs. 2A and 2B are explanatory fingers referred to in the discussion of Fig. l and other figures;

Fig. 3 is a schematic diagram of a velocity-modulation tube embodying the invention;

Fig. 4 is a schematic diagram of a modification of the tube of Fig. 3;

Fig. 5 is an explanatory figure referred to in the discussion of Figs. 3 and 4;

Fig. 6 is a sectional view taken on line 66 of Fig. 3;

Fig. 7 is a schematic diagram of a multicell modification of Fig. 4;

Fig. 8 is a longitudinal sectional view of a high-power multicell tube embodying the invention;

2,728,019 Patented Dec. 20, 1955 ice Figs 9 and 9A are detail views of structure shown in Fig. 8; and

Figs. 10 and 11 are detail views showing the internal construction of another multi-cell tube.

For a clear understanding of the invention, it is desirable first briefly to discuss the principles of operation of a prior conventional type of velocity-modulation tube.

Referring to Fig. 1, electrons leaving the cathode 10 move in a rectilinear path toward the collector or anode 15 with an initial velocity v0, corresponding to an accelerating voltage V0. As they pass through the velocitymodulating region A, defined in Fig. l by the grids 11 and 12 of an input cavity resonator 13, the electrons are subjected to a radio-frequency input voltage V equal to V1 sin wt, where V1 is the maximum value and w is the radian frequency of the input voltage, and I is the time of entry measured from the beginning of a cycle. After traversing the velocity modulating region each electronhas energy eVo+eV1 sin wt, where e is the electronic charge. The speed or velocity of the electron is now:

As the transit time for the electrons through the region A is short compared to the period of the input voltage wave, substantially all electrons entering the region A from the cathode side emerge from the opposite side and with the velocity v given by Expression 2. v

As indicated in Fig. 2A, the electrons which traverse the region A during one-half cycle have their velocity increased, whereas those which traverse the region A during the succeeding half cycle have their velocity decreased, the velocities of the successive electrons varying in accordance with the waveform of the applied voltage V. Otherwise stated, the electric field E1- is alternately accelerating and retarding with corresponding etfect upon the velocity of successive electrons. The electrons leaving region A during the intervals X of the period of voltage V sufier little acceleration or deceleration. Thus, the electrons which successively leave the region A in a cycle of frequency or move at correspondingly higher and lower velocities through a drift region B with the consequent bunching which is characteristic of velocity-modulation tubes. Thus, in the drift region B those electrons which have been decelerated in region A lag behind, and those which have 7 been accelerated in region A overtake the preceding electrons, to produce bunches of electrons or regions of high electron density along the beam path separated by regions of low electron density.

As each of the electrons passes through the catcher region C, defined in Fig. l by the grids 16-17, it excites the output cavity resonator 18 by induction to provide radio-frequency output power. The waveform of the collector current, as generally shown in Fig. 2B, comprises a series of sharp pulses P, one for each cycle of the velocity-modulating voltage V, arid each corresponding with thearrival of a bunch of electrons at the collector 15 Thus, in the drift region B, the beam current changes in form or Character from a velocity-modulated beam to a current-intensity-modulated beam whose pulses-'P, when suitable coupling-is provided to an output circuit, pro ducc radio frequency power of frequency ml 7 With such a conventional typeof velocity-modulation In general, such non useful and harmful electronsare these arriviiig at the collector during the intervals Z, Fig.

' It is afiobjectot the present invention to remove such improperly timed electrons from the beam, by periodically deflecting the beam and intercepting theimproperly timed electrons to preclude their passage through the catcher region C and to thecollectorelectrode 15.

Referring to Fig. 3','as eienip'lary' of one method and arrangement for accomplishing this object, the electrons from cathode 10 first pass through the velocity-modulating region A, and are then subje'cted;to a deflection field of thesame frequency as that of the velocity-modulating field but at right angles thereto, i. e., transverse to the direct path from cathode 10 to collector 15. Specifically, in Fig. 3, the velocity-modulated beam passes through velocity modulation grids 11 and 12 and then passes between deflection electrodes 19 and 20 interposed between thethe deflecting region D, there is imparted to them a lateral displacement y, defined by 3 :11 sin (win-{11) where d is a function ofthe electron transit time,

L 1=: "0 through the deflection region of length L withv initial velocity vo, andthe transverse electric field,

producedby a voltage Vi between plates having a spacing Safio-is the time of entry of'the electron into thedefiection region; and er-is a function of L, v0 and to.

Preferably, the electron beam is substantially rectangular in cross section produced by the emitting surface of thecathode 10 having its larger dimension perpendicular tothe plane of Fig. 3. Thus, in the transverse sectional view of- Fig. 6, the beam is shown in dotted lines as a rectangle of substantial area which periodically is sweptu'pand downacross the exit end of the deflection regiontube, there is provided in advance of the drift region B,

as at the exit endof the-deflection region D, a partial barrier electrode 21 which leaves a slit c positioned to allow passage only of the electrons which \vill b'urIch in the drift space to increase the tube output. The electrons which are improperly timed or phased-for bunching are intercepted by the barrier electrode 21 and, therefore,-

neitherabsorb energy from the catcher region (3 noradd to the collector current. Thus, as'shown in Fig. 5*"(i'n 4 contrast with Fig. 2B) the collector current of the tube of Fig-3 consists substantially only o'tpul'sesP', there being little or no flow of current in the intervals. In consequence, the output efiiciency of the tube is substantially higher than 5 8 per cent and approximates that of a class C amplifier or oscillator at low frequencies.

In the modification shown in- Fig. 4, the electron beam leaving the cathode 10 with a' velocity v0, passes first through thedeflection region D and thence through the velocity-modulating region A. As the beam passes;

through the deflection region D, it is acted upon by a transverse electric field V1 Sin calf in a direction perpendicular to v0. All ofthe electrons have essentially the same transit time, -r, through the deflection region D and, in consequencethey reach the velocity-modulating region A at the exit end of the deflection region D with a lateral displacement y, defined by y=d sin (wto+e2) (4) where d and 62 are constants which are determined by the transit'time electrons traversethe deflecting region in essentially. the

sameelapsedtime, or 62 is the same for all values of to.

By adjustingor selecting the parameters vn and L, two barrier electrodes 21 aud22rcaube so placed that when the maximum number of electronspassthrough the velocitymo dulating regionA, the phase of. the velocity-modulating field; is proper, for. bunching of those electrons attimes just preceding their, passage through the catcher region G. The, electrons, which pass out of the deflection region D later incach cycle are cutoff by the barri er electrodes 21; anddo not enter the velocity-modulating-region. Thus, the electrons which are not properly timed-orphasedto contribute tothe radio-frequency output of-the tubeare prevented from entering the driftspace and, are thusprecludcdfrom lowering the output efficiency of the-tube.

With both of Figs. 3 and 4, the pulse shape of the current entering and. leaving. region A is determined by sweeping a, beam, Fig. 6,.of width b with a deflection d sinwt acrossa slitofrwidth c properly positioned relative to the ,undefiected beam position. The slit should pass all of the available current whend sin wt=:d, depending upon the,phaseof: the field in the velocity-modulating region. A, because the, sweep speed is zero when sin wtEil. Thephase of the velocity-modulating field should be such that maximum current enters the velocitymodulatingregion when the-field passes through zero from a retarding field. The significant point is that-all elcctrons which reach the catcher region C are bunched" and that;electronsnotcontributing to useful R. F. output do noLreach. the catcher region because of their interception either at the entrance slit of the velocity-modulating region, Fig. 3, or at the exit slit. of the velocitymodulating region, Fig. 4.

For high power, tubes utilizing the invention, it is de sirable that the; collector current be, divided into several parallel beams, each in a separate cell defined by walls 14, 19A and, 20A. to avoid space-charge difficulties, as shown in the simple schematic arrangement of Fig.7. Thecurrent of collector 15A is that of beams respectively extending from the corresponding cathodes 10A to the common collector 15A. Each of the beams, as in Fig. 3 or Fig. 4, successively traverses a, vclccity-modulating region and a deflection region-before entry intoits drift region. In the particular arrangement shown in Fig. 7

fiection region D1 or D: and s'ubsequently'traversesa velocity-rnodulating region A1 or A2 and a drift region B1 or B2. The cell walls 14, 19A and 20A fix the space potential along the individual beams, thus preventing the electrons from lowering the space potential. This would not be true if the total beam current were concentrated in a single beam, since the current density of a single beam would be so high as to lower the space charge potential.

For modulating the radio-frequency output of any of the tubes of Figs. 3, 4 and 7, at audio, video, or other signal frequencies, there may be provided a control grid 23 between the cathode and the first of the regions A and D. The tube may also, as shown in Fig. 7, be provided with a screen-grid 24 between the control grid 23 and the first of the regions A and D.

In the structural example of Fig. 7 which is shown in Figs. 8, 9 and 9A, a multiplicity of cells are disposed in an annular array about the longitudinal axis 25 of the tube for high-power operation as an oscillator or amplifier. The particular tube structure shown in Fig. 8 is designed for operation at frequencies in the neighborhood of 1,000 megacycles and for delivering about 50 kilowatts of power at that frequency.

The annular collector 15A common to all cells is supported within the tube envelope by lead-in conductors 26, which may be tubular or hollow for the flow or a cooling fluid through the collector. At points remote from the collector, the conductors 26 are hermetically sealed to the free ends of metal sleeves 27 which include insulating rings or collars 28. The sleeves 27 and the portions of the conductors 26 enclosed thereby are externally bypassed to act as radio-frequency shorts. The sleeves 27 extend from and are hermetically sealed to a bell-shaped end member 29, whose right-hand end is closed by the dome or disk of insulating material. The periphery of the collector 15A and the adjacent rim area of hell member 29 serve as, or form, a by-pass condenser.

The drift member 31, apertured to provide a plurality of angularly-spaced drift regions B respectively in alignment with cathodes 10A of the tube, is provided with an axial supporting member 32 which passes through the large central opening of the collector electrode 15A and through the opening in the insulating disk 30 to which latter it is hermetically healed. The periphery of the drift member 31 closely fits the tubular member 33, which at its righthand end is hermetically sealed to the outer edge of an insulating ring 34 whose inner edge is hermetically sealed to the rim of the bell member 29.

The envelope member 33 should be of a material, such as Kovar, having a coeflicient of expansion such that it 1 can be sealed to the insulating ring 34, which would ordinarily be glass. To facilitate making this seal, and to protect the seal during heating of the tube during the exhausting operation, the drift member 31, which would ordinarily be made of copper, is provided with an annular recess 31a, to space the end of member 33 from the drift member 31. The outer end of recess 31a may be shorted by spring contacts 31b.

The drift member 31 is thus spaced, axially of the tube, from the collector electrode 15A to provide a plurality of catcher regions C therebetween. An output cavity resonator 18A common to all of the catcher regions may be slipped over the collector end of the tube, with its horizontal flange engaging the envelope member 33 and its vertical flange engaging the larger diameter base section of bell member 29.

At the opposite end of the tube, the envelope member 33 receives and is hermetically sealed to an end-cup member 35, from which extend a plurality of sleeves 36 having insulating rings 37 interposed between their free ends and the ends sealed to member 35. At their free ends, the sleeves 36 are sealed to cathode conductors 38. The sleeves 36 and the enclosed portions of the conductors 38 may be dimensioned to serve as radio-frequency bypasses. At least one of the cathode conductors 38 is 6 connected to and supports a ring conductor 39 to which one end of each of the cathodes 10A, in the form of a strip or ribbon, is attached: at least one other of the conductors 38 is connected to and supports a smaller ring conductor 40, to which the opposite ends of the cathodes 19A are attached. There is thus provided a spoke array of wide ribbon cathodes each aligned with a corresponding one of the drift passages B through drift member 31'.'

A control grid 23A, preferably of annular shape, is supported at its outer and inner rim portions by conductor rings 41 and 41a, which in turn are supported by insulating members (not shown) from envelope member 33 or a lead or leads through 33. A screen grid 24A is preferably also of annular shape with its inner and outer edges respectively attached to two metal rings 42, 43. The inner ring 42 is joined to or integral with a tube 44 which extends through and is sealed to the cup member 35. The outer or left-hand end of tube 44 is sealed to an insulating disk 45.

The deflection electrodes 19A and 20A are in a circular array, one pair per cell, the inner ends of electrodes 19A being attached (Figs. 8 and 9) to a supporting tube 46 and the inner ends of electrodes 20A being attached to a supporting tube 47. As shown in Fig. 8, the tubes 44, 46 and 47 are concentrically mounted, with insulating disk 45 spacing the tubes 44 and 46, and a disk 48 of insulating material spacing the tubes 46 and 47. As best shown in Fig. 9A, each electrode 19A serves as the electrode of two adjacent cells. Similarly each electrode 20A serves as the electrode of two other adjacent cells. At the exit end of the deflection regions, the ends of the deflection electrodes 19A and 2ilA are inturned, at 21A and 22A, respectively, in spaced planes normal to the axis of the tube, to provide combination velocity-modulation and partial barrier electrodes. The inturned portions 21A and 22A are provided with aligned apertures C1 and C2 permitting passage only of the electrons of the proper phasing for bunching as the beams traverse the subsequent catcher space C of the common output cavity resonator 18A.

The input coaxial line 49, for applying the deflection and velocity-modulation potential to the deflecting electrodes 19A, 20A and their velocity-modulation extensions 21A and 22A,is connected, as shown in dotted lines in Fig. 8, to the tubes 46 and 47 which serve, as above stated, as the conductive supports for the two series of electrodes 19A, 20A.

A closed-ended quarter-wave coaxial-line resonator 50, formed of tube 46 as the inner conductorand tube 44 with an external extension as the outer conductor, effectively decouples the screen grid 24A from its source of biasing voltage, so far as the operating frequency of the tube is concerned.

The operation of the tube of Fig. 8 should be clear from r the description of Figs. 2 to 7, particularly Fig. 7, and further explanation thereof appears unnecessary.

Another multi-cell arrangement in which the cells are disposed about the axis of the tube is shownin Figs. 10 and ll. In this modification, one series of deflection electrodes, such as 20B, is supported by attachment of their outer ends to an outer member 47B, which may be integral with or attached to the equivalent of envelope member 33 of Fig. 8. The other series of deflection electrodes, specifically 19B of Fig. 10, may be attached at their inner ends to a support tube 463. In such case, the input cavity or equivalent source of deflection and velocity-modulation potential will be connected across the members 46B, 47B. As shown in Fig. 10, each electrode 19B is provided with barrier electrodes 21B, leaving exit slits 0 each having the relation to a beam discussed in connection with Figs. 3, et seq. In the drift region, the cells may be rectangular in cross section, as

in Fig. 10, or they may be of keystone shape, as shown at 31B in Fig. 11, with radial cell walls 14B of substantially uniform thickness.

The:arrangementsofiFigs. wand; 11, like that of Fig. 8; are particularly suitedifor high: power tubes in that high, beam current without space charge limitation ispossible. In common with all modifications, they provide for output efliciencies far in excess of 58 per cent because they permit passage, to. the drift space of each beam, of those electrons whose timing or phase relation insures-their maximum bunching for contribution to the high-frequency. output, and-because they preclude, by the barriereslit arrangement, passage to the drift space of those; electrons which would otherwise absorb high-frequency power and increase the: direct-current component of the collector current;

In each ofthe forms of the invention shown in Figs. 3, 4, 7 and 8, suitable rneans-may-beprovided for establishinga constantmagnetic fieldzparallel to the axis of the tube, to focus theelectrontbearn or beams and prevent: spreading thereof. due tomutual repulsion of electrons. If such a magnetic field is provided, the lateral displacement of the electronsin the deflection region of. the tube-is also av function of the magnetic field strength.

It will be understood that. the invention is not limited tothe-particular tube structures, shown, and that modifications and changes may. bemade within the scope of the appended claims.

What is claimedis;

l. Avelocity-modulationtube comprising an elongated drift tube, means for projecting a beamof electrons along a. path extending therethrough, beam deflecting means and velocity modulating-means in the path of said beam between said first-namedmeansand said drift-tube, barrierl meansadjacent said drift tube adapted to permit entrance of the beam tosaid drift tube only during a predetermined part-of the deflection cycle, and output means in said beam path beyond said drift tube.

2; A velocityrmodulation tube as in claim 1, in which said velocity modulation means are located along the beam path in advance ofisaid beam deflecting means.

3. A velocity-modulation tube as in claim 1, in which said beam deflecting means are located along the beam pathin advance of said velocity modulation means.

4. A. velocity.modulation tube having means forming a-drift region, meansfor projecting a bearnof electrons through said drift-region, electrodes disposed in advance ofsaid drift region. forvelocity-modulating said beam prior to its passage through said drift region, deflection electrodes electrically connected to said velocity-modulating electrodes and disposed in advance of said drift region to cyclicallydefiect said electron beam, a partial barrier between saiddriftregion and said modulatingand deflection electrodes in position to preclude entrance to said drift regionof the-velocity-modulated beam for a predetermined part of the deflection cycle and output electrode meansdisposed in the path of the beam beyond said dri-ftregion.

5. A velocityemodulationtube as in claim 4, in which the. relative; positions of thevelocity-rnodulating and deflection electrodes along-the beam path are such that the beam is first. velocityemodulated and the velocity-modulatedibeam is then deflected.

6; A velocityemodulationtube as in claim 4, in which the relative positions of. the velocity=modulating and deflectionelectrodes alongthe beam path are suchthat the beam isfirst deflected, and after deflection is. velocitymodulated.

7.. A velocity-modulatioutube as in claim 4, in which said beam projecting means is shaped to provide a rib bon shaped beam, inwhich the field of said deflection. electrodes is normalto the width of the beam, and in Whi'chthe edge. of said-barrier. is parallel to the width of the beam.

.8. A- velocity-modulationtube having means forming a plurality of drift regionsyeach drift region having in advance thereof beam projecting means, velocity-modulat-I ingtand deflection electrodes, and a barrier, as in claim 4;

whereby high: beam current-density may be obtained-T without undue lowering of the space-charge potential along the beam path. 1

9. A high-power velocity-modulation tube comprising means including aplurality of cathodes for producing a corresponding number of electron beams; collector electrode means spaced from said cathodes for receivingsaid beams; and means, interposed between said cathodes and said collector electrode means, defining. a plurality of cells, one for each of said beams, each cell including a deflecting region, a velocity-modulating region, a drift region, and an output region for the beam, a pair of velocity-modulating electrodes in each.velocity-modulating.

region and a pair of. deflecting electrodes in each deflecting region, said velocity-modulating and deflecting electrodes being connected together for operation at the same signal voltage.

10. A velocity-modulation tube, as in claim 9, in which the cell defining means includes slit-forming structure positioned to permit passage or" each beam to its driftregion only during that part of its deflection cycle for which all the passed electrons are bunched when leaving said drift region during operationof said tube.

11. An electron tube comprising means including a plurali y of cathodescxtending radially of the axis of the tube and angularly spaced about said axis for providing a plurality of electron beams, a hollow. drift member spaced from said cathodes along said axis and providing a pinrality of drift regions respectively in alignment with said cathodes, a plurality of deflection electrodes extending radially of said axis andangularly spaced to provide between each cathode and the associated driit region a pair of deflection electrodes one on each side of the beam path from that cathode, and a pair of elocit wmodulation electrodes locatedin the path of the beam between each cathode and the associated drift region, each pair. of velocityrnodulation electrodes being connected tothe two deflection electrodes associated with the same beam path.

12. A tube asin claim 11, in which said deflection electrodes are alternately of two different lengths and have extensions normal to said axis to provide said pair of velocity-modulation electrodes for each of said beams;

13. A tube as in claim 12, in which said extensions include barrier portions adapted to permit passage of each beam to the associated drift region only when its electrons are of timingproper for benching at the exit from said drift region.

l4. A tube as in claim ll, further including a collector eiectrode spaced along said axis from said drift member and symmetrically disposed about said axis to receive all of said beams after their passage through said. drift member.

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